Crush a grape: Raman and FTIR seed phenols

Ezine

Published: Jul 1, 2017

Author: David Bradley

Channels: Raman

Phenolics

The extraction of phenols from grape skin and seeds has been optimized by researchers at the University of Seville, Spain and Dublin Technology Institute, Ireland. They used Raman and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopies to confirm the identities of the compounds remaining in the residue after processing.

The work published in Talanta by Julio Nogales-Bueno, Berta Baca-Bocanegra, José Miguel Hernández-Hierro, Francisco José Heredia of the Food Colour and Quality Laboratory, in the Department of Nutrition and Food Science, at Sevilla, Spain and Abigail Rooney and Hugh J. Byrne of the FOCAS Research Institute, at the Dublin Institute of Technology, in Dublin, Ireland, suggests that the squeezing of grapes has to be sufficiently intense to be able to extract the phenols from the internal layer of the grape skin. The work helps focus on how and why different approaches to grape pressing produces different extraction profiles in terms of phenols released during winemaking. The team hopes to provide evidence for how the extraction of such compounds might be improved given the purported health benefits of many of these compounds as well as their role in the overall quality of a given wine.

Organoleptics

"In the case of wine, phenols add organoleptic qualities of great importance, and on these the quality of the wine directly depends," explains Nogales-Bueno. "The appearance of these compounds in the wine mainly comes from the solid parts of the grape - skin and seeds - during winemaking," he adds. This fact was well known but now the team can relate the structural composition of the skin and the seeds with the ease with which we can obtain phenols and understand the differences in phenol extraction according to whether we use the external or internal surface of the skin.

"As the level of ripening of the grape increases, the cellular wall of the material of the berry starts to degrade and the phenols are released more easily," Nogales adds. "The external layer, called the cuticle, does not contain polyphenols and is mainly made up of hydroxyl fatty acids, while the layer closest to the pulp is where the majority of the phenols are found in the skin. Therefore, during the process of winemaking, the crushing stage has a great influence on the extractability of the phenols from the skin, as inefficient crushing does not generate enough contact between the grape-juice and the internal layer of the skin," he adds.

In terms of extraction from seeds, the diffusion of phenols decreases as the grape ripens because lignification occurs (sealing of the plant walls through the deposit of lignin, which makes it "woody") in much of the material, impeding the diffusion of phenols; as the authors explain in a paper published in the journal Food Chemistry.

Pectins and polysaccharides

The team investigated samples of red winemaking grapes from two plots in the Denominación de Origen Condado de Huelva during the 2014 and 2015 harvests. The skins and the seeds were separated in the samples to try to determine the ease of extraction of phenols from each of these sources. The researchers applied statistical analyses to the FTIR and Raman spectroscopic data to see reveal the any putative correlation between the structural composition of the non-extractable material and the ease of release of phenols as observed in the samples. It seems that there do indeed exist clear structural differences between the external and internal parts of the skin and that these differences are related to the quantity of phenols that can be extracted from each part of the grape.

The team adds that they were able to attribute the greatest differences in the spectra to the presence of pectins, monosaccharides, polysaccharides, lipids and waxes. These compounds are all components of the skin. The amount of polysaccharide present and the degree of esterification of any pectins significantly influences the extractability of phenols. A similar picture emerged with tests on grape seeds.

"When we know more about the extraction of phenolic compounds from the solid parts of the grape, we will study several forms to increase the amount of phenolic compound that we can extract from low extractability grapes," Nogales-Bueno told SpectroscopyNOW. "Moreover, we are applying vibrational spectroscopy to other oenological products and byproducts (chips, oak wood, grape pomace, etc) in order to study their properties."

Related Links

Talanta 2017, 167, 44-50: "Linking ATR-FTIR and Raman features to phenolic extractability and other attributes in grape skin"